Vanilla-fy

Mostly just removed NGDS dependencies from the extension and tried to abstract the Geoserver object a bit more.  Got the plugin interface working, so 'geoserver' and 'ogc_preview' can both be added into a CKAN *.ini file as 'ckan.plugins'.  Refactored some of the ogc-preview code as well to be compatible with CKAN v2.2; testing has only been done on CKAN v2.2.